Raffinose saccharides are a group of D-galactose-containing oligosaccharides of sucrose that are widely distributed in plants. Raffinose saccharides are characterized by having the general formula: [O-α-D-galactopyranosyl-(1→6)n-α-glucopyranosyl-(1→2)-β-D-fructofuranoside where n=0 through n=4 are known respectively as sucrose, raffinose, stachyose, verbascose, and ajugose. The biosynthesis of raffinose saccharides has been fairly well characterized [see Dey, P. M. In Biochemistry of Storage Carbohydrates in Green Plants (1985)]. The committed reaction of raffinose saccharide biosynthesis involves the synthesis of galactinol (O-α-D-galactopyranosyl-(1→1)-myo-inositol) from UDP-galactose and myo-inositol. The enzyme that catalyzes this reaction is galactinol synthase. The flux of carbon through this reaction is controlled by the concentrations of the two substrates for the enzyme. Thus, while they are not unique to the raffinosaccharide pathway, the enzymes which produce these substrates serve to limit carbon flux to the raffinosaccharides.
UDP-glucose 4-epimerase (EC 5.1.3.2) is also called UDP-galactose 4-epimerase. It is responsible for the interconversion of UDP-glucose and UDP-galactose. UDP-galactose is a precursor of galactolipids and cell wall polysaccharides. When transgenic Arabidopsis plants expressing the UDP-glucose 4-epimerase gene in sense or antisense orientation are grown in soil, no changes in morphology or relative amounts of different galactose-containing compounds are detected. When the plants are grown on agar plates in the presence of galactose, a decrease in enzyme activity and an increase in the UDP-galactose content correlates with a repression of growth while the UDP-glucose content does not change. Changes in the amount of galactose in the cell wall is detected in plants with low UDP-Glucose epimerase activity grown on galactose, while there is no change in the cellulose content of the leaves (Dormann and Benning (1998) Plant J. 13:641-652).
The activity of UDP-glucose 4-epimerase appears to be particularly limiting to carbon flux into the raffinosaccharide pathway, therefore further reduction of the activity of this enzyme by tissue- and temporally-specific gene silencing should greatly decrease the levels of raffinose and stachyose in seeds.
Changes in the expression of either UDP-glucose 4-epimerase will allow the modification of the carbohydrate metabolism in transgenic plants. Modification of the expression of UDP-glucose 4-epimerase may result in grains with reduced cell-wall constituents (fiber) and increased levels of starch. This trait will add value for feed, food, and industrial applications of the crops.